Pigmentary titanium dioxide materials are widely used as coloring agents in plastics, paints, inks, papers, and in many other materials. In plastics, pigmentary titanium dioxides impart color and provide UV protection. The UV protection provided by the incorporation of titanium dioxide can substantially improve the durability of a plastic material.
A need presently exists for titanium dioxide materials which are more readily dispersed in plastics. In order to reduce the weight and cost of plastic film products, for example, the plastics industry is seeking ways to reduce the thickness of plastic film products while maintaining the strength and integrity of such products. The dispersibility of the pigmentary materials used in such applications becomes more significant as film thickness decreases. When a titanium dioxide pigment is inadequately dispersed in a plastic film product, the presence of agglomerated masses of undispersed pigment material (i.e., nibs) can destroy the utility of the plastic film. Additionally, undispersed pigment material can stick to process rollers and/or die parts and thereby produce holes, rips, and/or tears in the plastic film product. Further, poorly dispersed pigment materials can quickly clog polishing screens and other such devices used in film forming processes.
Titanium dioxides are generally produced in two crystalline forms, anatase and futile. Rutile titanium dioxide is commonly produced from titanium halides (preferably titanium tetrachloride) using vapor phase oxidation processes. Examples of vapor phase oxidation processes are disclosed in U.S. Pat. Nos. 3,208,866 and 3,512,219. The entire disclosures of these patents are incorporated herein by reference.
In a vapor phase oxidation process, a titanium halide vapor reactant is oxidized using an oxygen-containing gas such as molecular oxygen, air, or oxygen enriched air. Various particle size control agents and/or rutilization agents are commonly added to the vapor phase oxidation system. Water vapor is commonly added, for example, to control nucleation and, therefore, product particle size. Aluminum chloride is commonly added in order to stabilize the crystalline matrix of the titanium dioxide product material and promote rutilization.
The aluminum chloride added to the vapor phase oxidation system is oxidized in the system to form alumina. Generally, the amount of aluminum chloride added to the oxidation system will be an amount sufficient to yield an alumina concentration in the oxidation system product in the range of from about 0.05 parts to about 10 parts by weight per 100 parts by weight of titanium dioxide contained in the reaction system product.
The reactants employed in a vapor phase oxidation process are typically preheated prior to being combined in the reaction chamber. Additional heat is preferably provided in the reaction chamber by (a) introducing a combustible gas (e.g., carbon monoxide, benzene, naphthalene, acetylene, anthracene, or the like) directly to the reaction chamber and/or (b) adding such combustible gas to one or more of the reactant streams. Combustible gas will preferably be added to the supply conduit for the oxidizing gas such that the combustible gas is burned (a) in the oxidizing gas supply conduit immediately before entering the reaction chamber and/or (b) in the region of the reaction chamber wherein the reactants are mixed.
The amount of oxidizing gas employed in the vapor phase oxidation process will preferably be an amount in excess of the stoichiometric amount required for the combustion of the combustible material, for the oxidation of the titanium halide reactant, and for the oxidation of any other oxidizable additives used in the vapor phase oxidation process.
The reaction effluent from the vapor phase oxidation system is preferably cooled immediately upon leaving the reaction chamber. Such cooling is commonly accomplished, for example, by mixing a cool gas (e.g., cooled chlorine effluent obtained from the reaction process) with the reaction effluent stream or by contacting the reaction effluent stream with water.
The titanium dioxide product produced in the vapor phase oxidation process is a solid, agglomerated particulate material. Typically, the particulate titanium dioxide product is recovered from the reaction effluent using cyclones, bag filters, settling chambers, or a combination thereof.
Heretofore, the crude, agglomerated titanium dioxide material recovered from the vapor phase oxidation system effluent has typically been processed by (1) dispersing the crude material in an aqueous medium using a dispersing agent (e.g., a polyphosphate); (2) thoroughly wet milling the material; (3) precipitating inorganic oxides (e.g., alumina and/or silica) onto the particle surfaces of the wet milled titanium dioxide material; (4) recovering the alumina and/or silica treated titanium dioxide material from the aqueous medium by filtering; (5) washing and filtering the recovered product to remove salts and impurities therefrom; (6) drying the washed product; and (7) grinding the dried product to a desired size using, e.g., a fluid energy mill.
The inorganic oxides (e.g., alumina and/or silica) deposited on the wet milled titanium dioxide material change the surface properties of the particulate material so that the material will flocculate. The flocculation of the particulate material allows the material to be recovered and washed using conventional vacuum-type and/or pressure-type filtration systems.
Unfortunately, the presence of added inorganic oxides on the surfaces of the processed titanium dioxide material reduces the dispersibility of the material in plastics. Generally, this result occurs because (1) the deposited inorganic oxides increase the surface area of the particulate material and (2) the deposited inorganic oxides are generally hydrophilic. In contrast to the deposited inorganic oxides, plastics are generally hydrophobic.
If such inorganic oxides are not deposited on the particulate material, it has heretofore been necessary to add polymeric flocculants and/or flocculating salts (e.g., magnesium sulfate) to the wet milled titanium dioxide dispersion in order to allow the milled material to be collected and recovered using conventional vacuum-type and/or pressure-type filtration systems. Unfortunately, such flocculating agents add undesirable impurities to the system and detract from the properties of the processed titanium dioxide product.